11 research outputs found
Electronic structure changes during the surface-assisted formation of a graphene nanoribbon
High conductivity and a tunability of the band gap make quasi-one-dimensional
graphene nanoribbons (GNRs) highly interesting materials for the use in field
effect transistors. Especially bottom-up fabricated GNRs possess well-defined
edges which is important for the electronic structure and accordingly the band
gap. In this study we investigate the formation of a sub-nanometer wide
armchair GNR generated on a Au(111) surface. The on-surface synthesis is
thermally activated and involves an intermediate non-aromatic polymer in which
the molecular precursor forms polyanthrylene chains. Employing angle-resolved
two-photon photoemission in combination with density functional theory
calculations we find that the polymer exhibits two dispersing states which we
attribute to the valence and the conduction band, respectively. While the band
gap of the non-aromatic polymer obtained in this way is relatively large,
namely 5.25 ± 0.06 eV, the gap of the corresponding aromatic GNR is strongly
reduced which we attribute to the different degree of electron delocalization
in the two systems
End states, band gap, and dispersion
Angle-resolved two-photon photoemission and high-resolution electron energy
loss spectroscopy are employed to derive the electronic structure of a
subnanometer atomically precise quasi-one-dimensional graphene nanoribbon
(GNR) on Au(111). We resolved occupied and unoccupied electronic bands
including their dispersion and determined the band gap, which possesses an
unexpectedly large value of 5.1 eV. Supported by density functional theory
calculations for the idealized infinite polymer and finite size oligomers, an
unoccupied nondispersive electronic state with an energetic position in the
middle of the band gap of the GNR could be identified. This state resides at
both ends of the ribbon (end state) and is only found in the finite sized
systems, i.e., the oligomers
Nonlinear optical response of photochromic azobenzene-functionalized self- assembled monolayers
The combination of photochromic and nonlinear optical (NLO) properties of
azobenzene-functionalized self-assembled monolayers (SAMs) constitutes an
intriguing step towards novel photonic and optoelectronic devices. By
utilizing the second-order NLO process of second harmonic generation (SHG),
supported by density-functional theory and correlated wave function method
calculations, we demonstrate that the photochromic interface provides the
necessary prerequisites en route towards possible future technical
applications: we find a high NLO contrast on the order of 16% between the
switching states. These are furthermore accessible reversibly and with high
efficiencies in terms of cross sections on the order of 10−18 cm2 for both
photoisomerization reactions, i.e., drivable by means of low-power LED light
sources. Finally, both photostationary states (PSSs) are thermally stable at
ambient conditions
Local resonances in STM manipulation of chlorobenzene on Si(111)-7×7: performance of different cluster models and density functionals
Reversible Photoswitching of the Interfacial Nonlinear Optical Response
Incorporating photochromic molecules
into organic/inorganic hybrid
materials may lead to photoresponsive systems. In such systems, the
second-order nonlinear properties can be controlled via external stimulation
with light at an appropriate wavelength. By creating photochromic
molecular switches containing self-assembled monolayers on Si(111),
we can demonstrate efficient reversible switching, which is accompanied
by a pronounced modulation of the nonlinear optical (NLO) response
of the system. The concept of utilizing functionalized photoswitchable
Si surfaces could be a way for the generation of two-dimensional NLO
switching materials, which are promising for applications in photonic
and optoelectronic devices
Electrocatalytic <i>Z</i> → <i>E</i> Isomerization of Azobenzenes
A variety of azobenzenes
were synthesized to study the behavior
of their <i>E</i> and <i>Z</i> isomers upon electrochemical
reduction. Our results show that the radical anion of the <i>Z</i> isomer is able to rapidly isomerize to the corresponding <i>E</i> configured counterpart with a dramatically enhanced rate
as compared to the neutral species. Due to a subsequent electron transfer
from the formed <i>E</i> radical anion to the neutral <i>Z</i> starting material the overall transformation is catalytic
in electrons; i.e., a substoichiometric amount of reduced species
can isomerize the entire mixture. This pathway greatly increases the
efficiency of (photo)switching while also allowing one to reach photostationary
state compositions that are not restricted to the spectral separation
of the individual azobenzene isomers and their quantum yields. In
addition, activating this radical isomerization pathway with photoelectron
transfer agents allows us to override the intrinsic properties of
an azobenzene species by triggering the reverse isomerization direction
(<i>Z</i> → <i>E</i>) by the same wavelength
of light, which normally triggers <i>E</i> → <i>Z</i> isomerization. The behavior we report appears to be general,
implying that the metastable isomer of a photoswitch can be isomerized
to the more stable one catalytically upon reduction, permitting the
optimization of azobenzene switching in new as well as indirect ways
Electrocatalytic <i>Z</i> → <i>E</i> Isomerization of Azobenzenes
A variety of azobenzenes
were synthesized to study the behavior
of their <i>E</i> and <i>Z</i> isomers upon electrochemical
reduction. Our results show that the radical anion of the <i>Z</i> isomer is able to rapidly isomerize to the corresponding <i>E</i> configured counterpart with a dramatically enhanced rate
as compared to the neutral species. Due to a subsequent electron transfer
from the formed <i>E</i> radical anion to the neutral <i>Z</i> starting material the overall transformation is catalytic
in electrons; i.e., a substoichiometric amount of reduced species
can isomerize the entire mixture. This pathway greatly increases the
efficiency of (photo)switching while also allowing one to reach photostationary
state compositions that are not restricted to the spectral separation
of the individual azobenzene isomers and their quantum yields. In
addition, activating this radical isomerization pathway with photoelectron
transfer agents allows us to override the intrinsic properties of
an azobenzene species by triggering the reverse isomerization direction
(<i>Z</i> → <i>E</i>) by the same wavelength
of light, which normally triggers <i>E</i> → <i>Z</i> isomerization. The behavior we report appears to be general,
implying that the metastable isomer of a photoswitch can be isomerized
to the more stable one catalytically upon reduction, permitting the
optimization of azobenzene switching in new as well as indirect ways